In Situ Construction of Bronze/Anatase TiO2 Homogeneous Heterojunctions and Their Photocatalytic Performances

Photocatalytic degradation is one of the most promising emerging technologies for environmental pollution control. However, the preparation of efficient, low-cost photocatalysts still faces many challenges. TiO2 is a widely available and inexpensive photocatalyst material, but improving its catalytic degradation performance has posed a significant challenge due to its shortcomings, such as the easy recombination of its photogenerated electron–hole pairs and its difficulty in absorbing visible light. The construction of homogeneous heterojunctions is an effective means to enhance the photocatalytic performances of photocatalysts. In this study, a TiO2(B)/TiO2(A) homogeneous heterojunction composite photocatalyst (with B and A denoting bronze and anatase phases, respectively) was successfully constructed in situ. Although the construction of homogeneous heterojunctions did not improve the light absorption performance of the material, its photocatalytic degradation performance was substantially enhanced. This was due to the suppression of the recombination of photogenerated electron–hole pairs and the enhancement of the carrier mobility. The photocatalytic ability of the TiO2(B)/TiO2(A) homogeneous heterojunction composite photocatalyst was up to three times higher than that of raw TiO2 (pure anatase TiO2).


Introduction
Environmental pollution is one of the biggest crises facing humans, and many environmental treatment technologies have been developed to control pollution [1,2]. Photocatalytic degradation has become one of the most promising emerging environmental pollution control technologies due to its mild reaction conditions, lack of secondary pollution generation, and environmental friendliness [3][4][5][6]. The key to the practical large-scale application of photocatalysis is the preparation of efficient, low-cost photocatalysts. However, most currently available photocatalysts fail to achieve a balance between a high efficiency and a low cost [7,8]. The modification of inexpensive, widely available photocatalysts to enhance their photocatalytic performances has become an important research direction for photocatalytic degradation technology [9][10][11]. TiO 2 is a non-toxic, non-hazardous, and low-cost photocatalyst with stable performance, but it exhibits poor photocatalytic performance, which must be enhanced to meet the needs of practical applications [12][13][14][15][16]. The low performance of TiO 2 photocatalysts is attributed to the easy recombination of photogenerated electron-hole pairs and the difficulty of visible light absorption. The construction of a homogeneous heterojunction is an effective method for suppressing the recombination of photogenerated electron-hole pairs and enhancing the carrier mobility, thereby improving the photocatalytic performance of photocatalysts [17][18][19][20][21][22]. As a rare phase in TiO 2 , TiO 2 (B) (bronze TiO 2 ) has a good electrical conductivity, a loose, porous structure, and it can be constructed with TiO 2 in the anatase phase (denoted as TiO 2 (A)), to form a homogeneous heterojunction crystal lattice with very small mismatches. As a result, the TiO 2 (B)/TiO 2 (A) homogeneous heterojunction will exhibit good photocatalytic degradation performance [23][24][25][26]. In this study, TiO 2 (B)/TiO 2 (A) homogeneous heterojunctions were constructed in situ by a simple hydrothermal method, utilizing TiO 2 (A) as the raw material with ion exchange and calcination. The prepared TiO 2 (B)/TiO 2 (A) homogeneous heterojunction material exhibited an excellent photocatalytic degradation performance, which was up to three times higher than that of raw TiO 2 . At present, the research on TiO 2 homogeneous heterojunctions mostly focuses on anatase/rutile TiO 2 , while much less research focuses on bronze/anatase TiO 2 . This study will provide new ideas and beneficial attempts for the research of TiO 2 homogeneous heterojunctions.

Materials
The rhodamine B (RhB, C 28 H 31 CIN 2 O 3 , Purity: 95%) was purchased from china Beijing Solarbio Technology, and both the sodium hydroxide (NaOH, Purity: 96%) and nano titanium dioxide (TiO 2 , Purity: 99.8%) were purchased from china Shanghai Aladdin Industrial Company. All reagents used in the experiments were of analytical grade and were used without further purification.

Preparation of Samples
A 100 mL 10 M NaOH aqueous solution was prepared, and 1 g raw TiO 2 was added. The mixture was stirred well, poured into a 150 mL hydrothermal reactor, heated to 220 • C in a blast drying oven, and held for 8 h. The reaction product was taken out and rinsed with a large amount of deionized water to near neutrality, after which it was dried at 60 • C for 10 h to obtain an intermediate product, which was labeled DNaT-220. The DNaT-220 was then soaked in 1000 mL 0.1 M dilute hydrochloric acid for 72 h, rinsed with a large amount of deionized water to near neutrality, dried at 60 • C for 10 h, and finally calcined in a muffle furnace at 500 • C for 2 h with a heating rate of 5 • C/min to obtain the final product, which was labeled DT-220. The intermediate products , and DNaT-160 as well as the final products DT-200, DT-190, DT-180, and DT-160 were prepared in the same way by changing only the temperature of the hydrothermal reaction. The number after the hyphen indicates the reaction temperature in • C.

Analysis and Testing
The phases of the samples were tested and analyzed using X-ray powder diffraction (XRD, D8 Advance, Bruker, Bremen, Germany) operated with a Cu target, wide-angle diffraction, a scanning range of 10-70 • , a scanning rate of 10 • /min, a test wavelength of 1.5406 Å, a tube voltage of 40 kV, and a tube current of 40 mA. The morphologies and structures of the samples were observed and analyzed using a field-emission scanning electron microscope (FE-SEM, Gemini SEM 300, Zeiss, Oberkochen, Germany) operated at a working voltage of 3 kV. The morphologies and structures of the samples were also observed and analyzed using a field-emission transmission electron microscope (Talos F200X, FEI, Hillsboro, OR, USA) operated at a working voltage of 200 kV, with a maximum magnification of 1.1 million times and a limit resolution of 0.12 nm. The photocatalytic degradation performance for RhB and the ultraviolet-visible (UV-vis) absorption spectra of the samples were tested by UV-vis spectrophotometry (UV-3600, Shimadzu, Japan) in the wavelength range of 200-2500 nm. The photocurrents and impedances of the samples were tested using an electrochemical workstation (CHI-760E, Chinstruments, Shanghai, China).
To test the photocatalytic degradation performance, 2 L of 20 mg/L RhB solution was prepared. In each experiment, 100 mL of RhB solution was added to 20 mg of the sample, and the mixture was sonicated for 30 min. Then, the mixture was placed under the light source of a solar simulator (Solar-500Q, Newbit, Beijing, China) with an intensity of 600 W/m 2 (at the liquid surface), and the photocatalytic degradation experiment was performed for 60 min. Every 10 min, 5 mL of liquid was taken from the mixture and placed in a centrifuge for centrifugation at 10,000 rpm for 10 min. After this, the supernatant was removed, and the absorbance (A n ) was measured by UV-vis spectrophotometry. The solution concentration (C n ) was proportional to A n , and thus, C n /C 0 = A n /A 0 , where A 0 is the absorbance of the 20 mg/L RhB solution. Figure 1a shows the XRD patterns of the raw TiO 2 and the intermediate products DNaT-220, DNaT-200, DNaT-190, DNaT-180, and DNaT-160. The raw TiO 2 diffraction peaks matched well with the standard PDF card JCPDS#21-1272, which corresponds to the most common anatase TiO 2 phase. The XRD patterns of the DNaT-220 and DNaT-200 had identical peak positions and shapes, coinciding with the PDF card JCPDS#31-1329, which indicated that both samples contained the same Na 2 Ti 3 O 7 phase. The peaks of the samples with lower hydrothermal reaction temperatures, DNaT-190, DNaT-180, and DNaT-160, changed significantly, which mainly manifested as increases in the half-peak widths of the spectral lines. All the samples contained Na 2 Ti 3 O 7 . However, the positions of the strongest and the second strongest peaks (corresponding to the TiO 2 in the anatase phase) were close to those of the peaks of Na 2 Ti 3 O 7 , which were relatively strong. Therefore, the peaks of the anatase TiO 2 phase (corresponding to the incompletely reacted raw TiO 2 ) were not clearly observed.    Figure 1b shows the XRD patterns of the raw TiO 2 and the final products-DT-220, DT-200, DT-190, DT-180, and DT-160. The peak positions and shapes of the DT-220 and DT-200 spectra were identical, and their standard PDF card number was JCPDS#35-0088, which corresponded to the bronze TiO 2 phase. When the hydrothermal reaction temperature dropped to 190 • C, the diffraction peaks of the bronze TiO 2 phase (corresponding to the product DT-190) were significantly weakened, whereas significant diffraction peaks of the anatase TiO 2 phase were observed. As the hydrothermal reaction temperature continued to be reduced, only the diffraction peaks completely consistent with those of raw TiO 2 were observed in the XRD patterns of the products DT-180 and DT-160. The TiO 2 ·xH 2 O produced by the generated Na 2 Ti 3 O 7 via ion exchange was converted to TiO 2 (B) after calcination at 500 • C.

Results and Discussion
The above phase analysis revealed that the entire phase-transformation process under the present experimental conditions was as follows: At hydrothermal reaction temperatures of 220 • C and 200 • C, the raw TiO 2 was completely converted by hydrothermal reactions to Na 2 Ti 3 O 7 , which then led entirely to the product TiO 2 (B) after ion exchange and hightemperature calcination. At a hydrothermal reaction temperature of 190 • C, the raw TiO 2 was partly converted via a hydrothermal reaction to Na 2 Ti 3 O 7 , and then ion exchange and high-temperature calcination led partly to the product TiO 2 (B) and partly to the incompletely reacted anatase TiO 2 phase. With a further decrease in the hydrothermal reaction temperature, the anatase phase TiO 2 was converted to Na 2 Ti 3 O 7 at a slower rate, the proportion of Na 2 Ti 3 O 7 converted by the reaction further decreased, and the amount of the remaining unreacted anatase TiO 2 phase further increased accordingly. Through controlled hydrothermal reactions, the structure of the raw TiO 2 could be easily modulated between anatase and bronze phases, providing a method for homogeneous heterojunction formation in situ, as well as electrical property adjustment. Combined with the XRD analysis results, we concluded that the band-like material in the two sets of SEM images was Na 2 Ti 3 O 7 (Figure 2a,b), which retained this morphology after ion exchange and calcination (Figure 2e,f) but was converted to TiO 2 (B). These band-like Na 2 Ti 3 O 7 and TiO 2 (B) were also observed in samples DNaT-220 and DT-220, respectively. When the hydrothermal reaction temperature dropped to 190 • C, a three-dimensional network started to appear in the DNaT-190 and DT-190 samples and many fine particles were dispersed on the network; these were TiO 2 (A) crystals that had not been converted. As the hydrothermal reaction temperature was further reduced, the conversion rate of the raw TiO 2 to Na 2 Ti 3 O 7 decreased. When the hydrothermal reaction temperature decreased to 180 • C, band-like Na 2 Ti 3 O 7 could no longer be generated by the reaction. Morphologically, the product consisted of Na 2 Ti 3 O 7 with a network structure and granular TiO 2 (A) dispersed in the network structure that had not completely reacted, as shown in Figure 2c Figure 4a shows the photocatalytic degradation curves of the raw TiO 2 , DT-200, DT-190, DT-180, and DT-160, respectively (Each sample was tested three times and the average value was taken). The photocatalytic degradation performances of the two-phase composite products, DT-190, DT-180, and DT-160, composed of TiO 2 (B) and TiO 2 (A) were significantly higher than those of the raw TiO 2 and pure TiO 2 (B). The DT-180 significantly outperformed the DT-190 and DT-160, and the photocatalytic degradation performance of the DT-180 heterojunction composite was nearly three times higher than that of raw TiO 2 . In addition, we can see that the samples had a small amount of adsorption on RhB during 30 min of ultrasound. Figure 4b quantitatively compares the catalytic efficiencies of the above samples, according to the kinetic calculation results. In terms of the Langmuir-Hinshelwood model, degradation can be considered as a first-order reaction at low RhB concentration. Therefore, the fitting calculation of degrading kinetics can be simplified to be linear, and this reaction satisfies the apparent first-order reaction rate equation: ln(C/C 0 ) = −k 1 t, where k is the apparent rate constant of the first-order reaction, and ln(C/C 0 ) is a function of irradiation time t. The calculated k values of no catalyst, Raw TiO 2 , DT-200, DT-190, DT-180 and DT-160 are 0 min −1 , 0.004 min −1 , 0.006 min −1 , 0.013 min −1 , 0.020 min −1 and 0.014 min −1 , respectively. These results suggest that the ratio of TiO 2 (B) to TiO 2 (A) in the product was related to the photocatalytic performance of the TiO 2 (B)/TiO 2 (A) homogeneous heterojunction composite photocatalyst, and that the composite exhibited the best photocatalytic performance when the optimal ratio was approached. In addition, the recyclability of DT-180 in RhB degradation reaction under UV-Vis irradiation was investigated, as shown in Figure 4c. After three successive cycles, the DT-180 sample retained nearly consistent photocatalytic efficiency, indicating that the TiO 2 (B)/TiO 2 (A) homogeneous heterojunction composite is stable during the degrading process.  Figure 5a shows the UV-vis absorption spectra of the raw TiO 2 , DT-200, and DT-180. Compared with those of raw TiO 2 and pure TiO 2 (B), the light absorption performance of the TiO 2 (B)/TiO 2 (A) homogeneous heterojunction composite photocatalyst in the UV region was not improved, but rather decreased due to the successful construction of homogeneous heterojunctions. The light absorption performance in the visible region was not significantly improved, with only a very slight red-shift. Figure 5b shows the Tauc plot of the raw TiO 2 , DT-200, and DT-180. The band gap energy of the semiconductors can be obtained from the x-axis intercept of the extended line. From the graph, the band gap energy values for the above samples were determined to be 3.33 ev, 3.18 ev, and 3.24 ev, respectively, which suggests that the band gap energy of TiO 2 (A) was reduced after introducing TiO 2 (B) to TiO 2 (A) to form the homogeneous heterojunction composite.  To investigate the origins of the improved photocatalytic performance, the light responses and electrical properties were measured. Figure 6a shows the transient photocurrent responses of the raw TiO 2 , DT-200 (representative of TiO 2 in the bronze phase), and DT-180. The photocurrent of DT-200 was higher than that of the raw TiO 2 in the anatase phase by a factor of approximately 1.5, and that of DT-180 was significantly higher than that of DT-200, by a factor of more than 1.5. Usually, a higher photocurrent indicates a weaker recombination of photogenerated electrons and holes. Thus, the recombination of photogenerated electron-hole pairs was significantly suppressed when the structure of TiO 2 was transformed from the anatase phase into the bronze phase. This suggests that TiO 2 (B) exhibited a slower recombination rate of photogenerated carriers than TiO 2 (A). In addition, the formation of TiO 2 (B)/TiO 2 (A) homogeneous heterojunctions further boosted the photogenerated charge separation because of the small built-in potential [25,27], leading to the substantially increased photocurrent shown in Figure 6a. Figure 6b shows the electrochemical impedance plots of the raw TiO 2 , DT-200, and DT-180. Generally, a larger arc radius indicates a higher impedance of the sample. Therefore, it was concluded that the impedance of DT-200 was lower than that of the raw TiO 2 , while that of DT-180 was much lower than those of both the raw TiO 2 and DT-200. This indicated that the conductivity and carrier mobility were improved by the transformation of TiO 2 (A) to TiO 2 (B) and were further enhanced by the formation of TiO 2 (B)/TiO 2 (A) homogeneous heterojunctions. Overall, the construction of TiO 2 (B)/TiO 2 (A) homogeneous heterojunctions not only benefitted the charge separation in the photocatalytic process, but also improved the electrical characteristics. The synergistic effects generated excellent photocatalytic activities, showing great potential for low-cost and high-efficiency photodegradation catalysts in environmental applications.

Conclusions
In this study, a novel photocatalyst consisting of TiO 2 (B)/TiO 2 (A) homogeneous heterojunctions was successfully prepared through facile hydrothermal processes. The structural characterizations revealed that a transformation occurred in the structure from the anatase phase to the bronze crystal phase, during which a series of TiO 2 (B)/TiO 2 (A) homogeneous heterojunctions formed. No significant improvement in the light absorption was observed, except for a very small red-shift in the absorption spectra. However, a substantial increase in photocurrent was detected for TiO 2 (B) in the transient light response measurement compared to that of TiO 2 (A). Additionally, further growth of the photocurrent for the homogeneous heterojunction was clearly observed, which was likely due to the small built-in field at the heterojunction interface. Moreover, the electrochemical impedance test revealed that the TiO 2 (B)/TiO 2 (A) homogeneous heterojunction possessed a substantially higher conductivity than that of TiO 2 (A), which also benefited the photocatalytic activities. The results of this study also indicated that the photocatalytic degradation performance of the TiO 2 (B)/TiO 2 (A) homogeneous heterojunction composite photocatalyst can be further enhanced by improving its light absorption capacity. This work provides a new structural design concept and a simple preparation method for the development of low-cost and high-efficiency composite photocatalysts.